This invention relates to a current source which can be used in, for example, bipolar semiconductor integrated circuits.
Recently, semiconductor integrated circuits have been used in a variety of portable electronic equipments. Most of the portable electronic equipments have a battery for the power supply. The voltage between the terminals of the battery decreases as it repeatedly supplies its power. Even under this voltage-changing power supply, use of a current source which does not change its preset current has assured the performances of many portable electronic apparatus.
The current source of this kind, as disclosed in JP-A-60-191508, has a current mirror which is formed of first to third transistors of the same polarity and transistors of the opposite polarity, and resistors. In this case, the base current of the third transistor is set at a proper value in order to equalize the collector-emitter voltages Vce of the first and second transistors which are used as a reference for the current setting, and also to make their collector currents equal. Thus, the value of the current from this current source is not affected by a voltage change of the power supply, the temperature dependency of the current amplification factors hfe of the transistors and the dispersion between production lots.
The arrangement of such a current source will be described with reference to FIG. 7. Referring to FIG. 7, there are shown NPN transistors 1, 2, 3 and 8. The first transistor 1 has an emitter area equivalent to N second transistors 2 connected in parallel. There are also shown resistors 4 and 332, which are connected to the emitters of the first and third transistors 1 and 3, respectively. The collector current of the third transistor 3 flows to an input end of a current mirror 530 which is formed of PNP transistors 531 through 535. The collector current Ic.sub.531 of the transistor 531 flows in the opposite direction to the collector of the first transistor 1 having a diode configuration as the first output current. Similarly, the collector current Ic.sub.532 of the transistor 532 flows to the collector of the second transistor 2 as the second output current, and the collector current Ic.sub.535 of the transistor 535 to the collector of the transistor 8 of diode configuration, or a load as the third output current. There are also shown a phase compensation capacitor 7 for negative feedback stabilization, a resistor 333 through which a current necessary for starting flows, and a power supply 9.
The operation of this conventional arrangement will be described with reference to FIGS. 7 and 8. The base-emitter voltage V1 of the second transistor 2 shown in FIG. 7 can be expressed by the collector current Ic.sub.1 of the first transistor 1 and the collector current Ic.sub.2 of the second transistor 2 as in the following equations (1) and (2): EQU V1=Vt*In(Ic.sub.1 /(Is*N))+R4*Ic.sub.1 ( 1) EQU V1=Vt*In(Ic.sub.2 /(Is) (2)
where
Vt=kT/q PA1 k: Boltzmann's constant PA1 q: charge of electron PA1 T: absolute temperature PA1 Is: reverse saturation current of NPN transistor PA1 R4: resistance value of resistor 4 PA1 *: multiplication PA1 the coordinates of point P are (0,0), and PA1 the coordinates of point Q are (Vt*In(N)/R4, Vt*In((Vt*In(N)/R4)/Is)).
FIG. 8 shows the curves of each term of Eqs. (1) and (2) and V1 of each equation with respect to the collector current Ic.sub.1, Ic.sub.2 in the abscissa. The points P and Q in FIG. 8 are the intersections of Eqs. (1) and (2), which satisfy Ic.sub.1 =Ic.sub.2 and have the common V1. By simultaneously solving the equations (1) and (2), it is possible to obtain the coordinates (collector current, base potential V1) of these points as follows:
Therefore, from FIG. 8, it will be found that Ic.sub.1 &gt;Ic.sub.2 is satisfied when the magnitude of V1 is in the range from point P to point Q, and that Ic.sub.1 &lt;Ic.sub.2 is satisfied when it is in the range larger than point Q.
If the base currents of the transistors 1 and 2 are now neglected, in the circuit arrangement of FIG. 7 the collector current Ic.sub.531 of the transistor 531 as the output from the current mirror 530 becomes the collector current Ic.sub.1 of the transistor 1 having a diode configuration, and the collector current Ic.sub.532 of the transistor 532 as the output from the current mirror 530 flows in the node of point A. In addition, the reverse collector current Ic.sub.2 of the transistor 2 flows in the node of point A. Thus, the magnitude of the total current flowing in the point A is (Ic.sub.1 -Ic.sub.2).
When the magnitude of V1 is in the range from point P to point Q, the collector currents of transistors 1 and 2 satisfy the condition of Ic.sub.1 &gt;Ic.sub.2. The current flowing in point A is positive, thus increasing the base current of the transistor 3 connected to point A. This results in an increase of the collector current Ic.sub.3 which is the input current to the current mirror 530. At this time, the collector current Ic.sub.531 of the transistor 531 as the output current from the current mirror 530 is increased, and thus the collector current Ic.sub.1 of transistor 1 is also increased. Thus, as is clear from FIG. 8, the difference between Ic.sub.1 and Ic.sub.2 becomes small and the current flowing in point A decreases.
When the magnitude of V1 is larger than point Q, the collector currents of the transistors 1 and 2 satisfy the condition of Ic.sub.1 &lt;Ic.sub.2, and the current flowing in point A is negative, thus decreasing the base current of the transistor 3 which is connected to the point A, or reducing the collector current Ic.sub.3 as the input current to the current mirror 530. At this time, the collector current Ic.sub.531 of the transistor 531 as the output current from the current mirror 530 is decreased, and thus the collector current Ic.sub.1 of the transistor 1 is also reduced. Thus, as is evident from FIG. 8, the difference between Ic.sub.1 and Ic.sub.2 becomes small, and the current flowing in point A is decreased.
As the result of this operation, the circuit arrangement shown in FIG. 7 is stabilized at point Q. The output current at this operating point, for example, the collector current Ic.sub.535 of the transistor 535 as one output current from the current mirror 530 can be expressed by the following equation (3): EQU Ic.sub.535 =Vt*In(N)/R4 (3)
From FIG. 8, it will be found that there is another stabilization point P. The resistor 333 is provided so that even if the collector current of the transistor 3 is 0, the collector currents Ic.sub.1, Ic.sub.2 of the transistors 1, 2 are not 0, or the operation is not stabilized at point P.
In the above description, it is assumed that the current amplification factor hfe of each transistor is large and that the base current of each transistor can be neglected. However, the base current is temperature dependent and there is a large dispersion between production lots, thus degrading the precision of the apparatus output. Therefore, the collector current Ic.sub.3 of the transistor 3 is set to the sum of the collector currents of the transistors 1 and 2. In other words, a current value corresponding to the base current of transistor 1, 2 which is removed from the collector current Ic.sub.531 of the transistor 531 of the current mirror 530 is also removed from the collector current Ic.sub.532 of another transistor 532 of the current mirror 530. This means that the base current of the transistor 3 can be increased to twice that of the transistor 1 or 2 by setting the input current to the current mirror 530 at twice the output current. As a result, the collector currents Ic.sub.1 and Ic.sub.2 of the transistors 1 and 2 become equal.
In addition, since the collector-emitter voltages of the transistors 1 and 2 are equal independently of the power supply voltage, the early effect (the current amplification factor hfe depends on the collector-emitter voltage Vce) in the change of power supply voltage can be canceled out, and thus the output current is not easily affected by the change of power supply voltage.
Therefore, even the conventional current source can be prevented from being affected by the change of power supply voltage, the temperature dependency of hfe of a transistor and the dispersion between production lots.